Grain-oriented electrical steel sheets with electrical easy axes oriented in the rolling direction are being used as transformer core materials. As a method of production of such a grain-oriented electrical steel sheet, Japanese Patent Publication (B) No. 6-19112 discloses the method of irradiating a YAG laser substantially perpendicular to the rolling direction to introduce periodic linear stress fields in the rolling direction and thereby reduce the core loss. The principle of this method is that the closure domains formed due to the surface stress caused by scanning by the laser beam finely refine the 180° electrical domain wall spacing and that abnormal eddy current loss is reduced. This is called “electrical domain refinement”.
In the past, various methods have been disclosed regarding this technology. For example, Japanese Patent Publication (A) No. 6-57333 discloses a method of using a periodic-pulse CO2 laser, while Japanese Patent Publication (B) No. 6-19112 discloses a method of using a continuous wave YAG laser and defining the irradiated beam spot diameter, power, scan rate, etc. so that no surface scribing traces occur. In each method, it is disclosed that by limiting the irradiating conditions to specific ranges, the core loss improvement (i.e. reduction) is enhanced. These methods are currently being put to actual use. However, the need for reduction of the core loss of transformer cores remains high. Further, a method of producing low core loss electrical steel sheet at a high efficiency has been desired.
“Core loss” is mainly the total of the classical eddy current loss, abnormal eddy current loss, and hysteresis loss. Classical eddy current loss is loss substantially determined by the sheet thickness. The loss changing due to the laser electrical domain refinement is the abnormal eddy current loss and hysteresis loss. The closure domains imparted by laser electrical domain refinement finely refine the 180° electrical domain wall spacing and reduce the abnormal eddy current loss, but become factors causing the hysteresis loss to increase. Accordingly, forming closure domains as narrow as possible in the rolling direction keeps down the increase in the hysteresis loss and results in the total core loss becoming lower. However, if closure domains are too narrow, the effect of refining the electrical domains becomes small, As a result, as described in Japanese Patent Publication (B) No. 6-19112, when using a YAG laser, if making the irradiated beam spot diameter extremely small, even if adjusting the linear scan rate or laser power of the laser beam, there was the problem that a significant effect of improvement of the core loss could not be obtained.